Regulation concerning power factor correction for a wide range of devices is becoming increasingly stringent. For example, new regulation requires power factor correction for any light emitting diode (LED) power supply with output power higher than 5 W.
For low to medium power level (e.g., 5 W to 100 W), a flyback converter is often used. By forcing the average input current to follow the input voltage, high power factor can be achieved. In order to reduce the cost, critical conduction mode is often used to achieve power factor correction. However, this results in a ripple in the output voltage at harmonics of the line frequency. The second harmonic (e.g., 120 Hz for North America or 100 Hz for China, Europe) is of particular concern for DC lighting applications, such as LED lighting, as it results in visible flickering wherein the human eye can see fluctuation of the light emitting from the LED.
Despite concerns over visible flicker in the output light, LED lighting is becoming an important light source due to high lighting efficiency and long life span. A two-stage LED driver may achieve substantially flicker-free performance, but at the expense of lower efficiency and higher component cost. A single-stage LED driver is therefore desirable because of its high efficiency and low component cost.